1. Field of the Invention
The present invention relates to an image forming apparatus which forms a color image on a recording paper by multi-transferring toner images in respective colors on the recording paper based on color separated image signals.
2. Description of the Related Art
As an apparatus to form a color image on a recording paper by multi-transferring toner images in respective colors on the recording paper one over another, for instance, a quadruple tandem system copying machine is so far known. This type of copying machine has four image forming stations each of which forms a yellow, magenta, cyan or black color toner image, respectively.
In each of these image forming stations, an electrostatic latent image is formed on photosensitive bodies charged to prescribed potential based on color separated image signals and a toner image in respective color is developed and formed on the photosensitive bodies by supplying charged respective color toners to the electrostatic latent image.
Under the image forming stations, there is arranged a transfer belt which runs endlessly in contact with the photosensitive bodies of the stations. At the location in the inside of the transfer belt, opposite to the respective photosensitive bodies, transfer rollers are arranged. These transfer rollers are applied with transfer bias voltage to have electrostatic force act on toner images on the photosensitive bodies.
When a color image is formed using such the copying machine, a recording paper is conveyed between the image forming stations and the photosensitive bodies by holding it on the transfer belt. Toner images in respective colors are formed on the photosensitive bodies and prescribed transfer bias voltage is applied to each of the transfer rollers. When this transfer bias voltage is applied, an electrostatic force acts on the toner images formed on respective photosensitive bodies toward the transfer belt. By this electrostatic force, respective color toner images are multi-transferred on a recording paper. The color toner images multi-transferred on the recording paper are heated, pressed and fixed on the recording paper in the fixing device. Thus, when a toner image is fixed on the recording paper, a color image is formed on this recording paper.
In this type of copying machine, when transferring toner images in respective colors formed on respective photosensitive bodies on a recording paper, prescribed transfer bias voltage is applied to each of the transfer rollers. At this time, a relatively large electric field is formed by the white ground potential on the photosensitive bodies and the potential of the transfer rollers. Between a recording paper being conveyed between the photosensitive bodies and the transfer rollers and the photosensitive bodies, the discharge is produced by this large electric field and the recording paper is charged up by electric charge of polarity reverse to the polarity of the transfer bias voltage. Therefore, in the transfer process from the first station (yellow) to the fourth station (black), the transfer electric field is gradually reduced by the charge resulting from the charge-up of a recording paper. By this reduction of the transfer electric field, the faulty transfer of a toner image on a recording paper may be produced particularly in the fourth station.
Therefore, on a conventional copying machine it was so set that the transfer bias voltage was gradually increased toward the fourth station from the first station to prevent such the faulty transfer. However, depending on change in environmental conditions (temperature and humidity) and difference in type of recording paper, there is no margin of the transfer bias voltage particularly in the fourth station. Accordingly, the faulty transfer was produced under the environment of low temperature and low humidity and transfer pits were produced by excessive toner image transfer under the environment of high temperature and high humidity.
When outputting a monochromatic color image using the quadruple tandem system copying machine, the same transfer bias voltage as that when outputting the color image described above is also applied to the transfer rollers of respective stations.
Because of this, when particularly outputting only a black image of the fourth station, a recording paper was charged up likewise the above, causing the faulty transfer or producing transfer pits.